1. Technical Field
The present invention relates to a spectrum analyzer and a technique therefor. The spectrum analyzer generates a local signal, of which the frequency is swept, in a local signal generation section, inputs the local signal to a mixer together with an input signal, extracts a signal of a predetermined intermediate frequency band from the output from the mixer, detects the level of the extracted signal, and displays a spectrum waveform of a frequency component included in the input signal in a desired observation range. In addition, in the technique for the spectrum analyzer, the local signal generation section employs a PLL synthesizer (a fractional-N PLL synthesizer) configured to be able to minutely perform a frequency sweep by using a fractional frequency divider in a feedback loop between a VCO and a phase comparator, thereby preventing the unwanted waveform components caused by the spurious components of the PLL synthesizer from being displayed in measurement result.
2. Related Art
Generally, spectrum analyzers are used to detect an intensity of a signal component of an input signal included in a desired frequency range and display a waveform on a screen in which the horizontal axis represents the frequency and the vertical axis represents the intensity, and are thus configured as shown in FIG. 4.
That is, a local signal L subjected to a frequency sweep is generated by a local signal generation section 11, and is provided to a mixer 13 of a frequency conversion section 12 together with an input signal SIN. Then, by inputting the output thereof to a filter 14, a signal component SIF of a predetermined intermediate frequency band is extracted.
Here, with respect to a center frequency fIF of the intermediate frequency band, when an upper heterodyne receiver of which a local frequency fL is high is used, the local frequency fL may be swept and changed from fL1 to fL2. In this case, a component with frequency ranging from (fL1 fIF) to (fL2-fIF) among the components included in the input signal SIN is chronologically extracted. Then, the extracted signal SIF is input to a signal processing section 15, and an amplitude detection process and a band limiting process corresponding to a resolution bandwidth RBW are performed. By obtaining signal intensity data, that is, spectrum waveform data for every frequency and plotting the data in the frequency axis on the display section 16, it is possible to obtain a spectrum waveform with a frequency range of (fL1-fIF) to (fL2-fIF).
As described above, in the spectrum analyzers having a system that converts each frequency component of the input signal into the predetermined intermediate frequency band by using the local signal subjected to the frequency sweep, the frequency precision and reproducibility of the local signal is required. Hence, the PLL synthesizer is generally used.
As shown in FIG. 5, the PLL synthesizer divides the frequency of the output of a VCO (a voltage control oscillator) 11a by N through a frequency divider 11b, provides the output to a phase comparator 11c together with a reference signal R, and smoothes an error signal, which is output in response to the phase difference, through a loop filter 11d. Then, the PLL synthesizer provides the signal as a control signal (or as a part thereof) to the VCO 11a, and performs feedback control so as to make the phase difference equal to 0 or a regular value (π, π/2, or the like), thereby locking an output frequency fVCO of the VCO 11a to a product N·fR between a reference signal frequency fR and a frequency division ratio N of the frequency divider 11b. 
When using the PLL synthesizer having such a configuration as the local signal generation section 11 of the spectrum analyzer, it is necessary to continuously vary the frequency of the VCO 11a in a prescribed step. However, even in the case of the spectrum analyzer of which the observation range is as high as several GHz, the required frequency resolution is equal to or less than several Hz, and it is necessary for the resolution of the local frequency to be equal to or less than that.
In order to achieve the above-mentioned conditions by using the PLL synthesizer with the above-mentioned structure, it is necessary for at least one of the reference signal frequency fR and the frequency division ratio N to be minutely varied so as to change the output frequency fVCO in a step where it is equal to, for example, about several MHz.
Here, regarding the response speed of the PLL, as the reference signal frequency is higher, the band of the loop filter 11d can be set to be larger, and thus it is possible to perform a fast frequency change. However, the band widens by that amount, and thus C/N at a far position deteriorates. In contrast, when the reference signal frequency is set to be low, the C/N at the far position improves, but it takes time to perform the frequency change.
Accordingly, in the PLL synthesizer for which the fast frequency change referred to as the frequency sweep is necessary, it is necessary to minutely change the output frequency without lowering the reference signal frequency.
Further, since the loop gain is changed in accordance with the frequency division ratio, it is not a good idea to drastically change the frequency division ratio.
In other words, it is preferable to sweep the output frequency by setting the reference signal frequency to a certain high frequency and minutely changing the frequency division ratio in a certain range.
In the general integer frequency dividers, it is difficult to perform the frequency division of a signal of several GHz in minute steps. However, recently, fractional frequency dividers have been implemented, and thus by using them, it is possible to perform the frequency division of minute steps.
The PLL synthesizer using the fractional frequency divider is called a fractional-N PLL synthesizer, and is able to perform the frequency division of the frequency division ratio (N+F) with respect to the integer N and the value F (normally F is a fraction value) of 0 or more and less than 1. In principle, the frequency division of the frequency division ratio N and the frequency division of the frequency division ratio N+1 are performed at a certain ratio therebetween within a set length of time so as to make the average frequency division ratio equal to N+F.
For example, when N=100 and F=0.1, the frequency is divided by 100 9 times (the number of input pulses is 900), and the frequency is divided by 101 once (the number of input pulses is 101), and thus pulses of a divided frequency 10 times relative to a total of 1001 input pulses are output. Consequently, the average frequency division ratio is 1001/10=100.1.
For example, by setting the minimum number of digits of the value F to 10−6 or the like through the fractional frequency divider, the frequency of the VCO can be varied in minute steps. However, in the fractional frequency divider, spurious components (fractional spurious components), which are unavoidable in principle, occur.
The spurious components are caused by performing frequency division by integers N and N+1 with respect to the frequency division ratio (N+F). Thus, when the local signal includes such spurious components, the frequency differences between the input signal component and the spurious components may coincide with the intermediate frequency band. In this case, the input signal component is converted into the intermediate frequency band, and is thus displayed as a spectrum waveform, thereby performing erroneous measurement.
As a technique of removing the spectrum waveform caused by the fractional spurious components, in the following Japanese Unexamined Patent Application Publication No. 2008-111832, there is disclosed a technique that performs a sweep twice by changing the conditions at the time of the local signal sweep, for example, the frequency division ratio or the time constant of the low-pass filter, specifies the spectrum waveform caused by the fractional spurious components on the basis of the two spectrum waveforms which can be obtained by the sweep, and removes the spectrum waveform.